Electrophotography has come to be extensively applied in recent years not only in the field of copiers but also in the field of various printers and printing machines because of the excellent instantaneousness thereof, the ability thereof to give high-quality images, etc. As the photoreceptors, which are the nucleus of electrophotography, photoreceptors employing inorganic photoconductive materials such as selenium, arsenic-selenium alloys, and zinc oxide have been used hitherto. Recently, however, photoreceptors (organic photoreceptors) employing organic photoconductive materials which have advantages including causing no pollution, ease of film formation and production, and a high degree of freedom of material selection and combination are being mainly used.
Known as the layer configurations of organic photoreceptors are a so-called single-layer type photoreceptor in which a charge generation substance has been dispersed in a binder resin and a multilayer type photoreceptor in which a charge generation layer and a charge transport layer have been superposed. Photoreceptors of the multilayer type are frequently used because stable photoreceptors having high sensitivity are obtained by employing an optimal combination of a highly efficient charge generation substance and a charge transport substance and separately incorporating these substances into respective layers, and because there is a wide choice of materials to facilitate the regulation of properties.
In recent years, both copiers and printers are shifting from monochrome to full-color images. Methods for forming such a full-color image mainly include the tandem process and the four-cycle process, and methods of transfer to printing media include a direct transfer process, transfer drum process, intermediate-transfer process, multiple-development en bloc transfer process, and the like. Of these processes, the technique of color image formation by the tandem process, i.e., the process in which images of respective colors are formed in respective image-forming units and transferred successively, is an excellent process for image formation because there are a variety of recording materials usable therein, high full-color quality is attained, and full-color images can be obtained at high speed. Of these, the ability to give full-color images at high speed is an advantage not seen in the other processes.
As a result of the trends toward speed increase and full-color image formation in electrophotographic processes described above, durability in terms of repeated use has become essential to the properties required of the electrophotographic photoreceptor besides higher sensitivity and higher response speed. For providing an electrophotographic photoreceptor which satisfy these properties, it has become necessary to develop a high-performance charge transport substance which has a high mobility and shows a sufficient residual potential after exposure. Investigations on charge transport substances which have a tetraphenylbenzidine framework substituted with a styryl group or the like and in which the π-electron system has been thereby expanded are being made enthusiastically in order to overcome those problems, and many reports have been made thereon. (See, for example, patent documents 1 to 4).
The photosensitive layer of an electrophotographic photoreceptor which employs organic materials is obtained by dissolving a charge transport substance, a binder resin, etc. in a coating-fluid solvent and applying and drying the coating fluid obtained. The property required of the charge transport substance when this electrophotographic photoreceptor is produced is solubility in the coating-fluid solvent to be used for producing the coating fluid. In case where the charge transport substance has low solubility in the coating-fluid solvent, it may be impossible to dissolve a desired amount of the charge transport substance in the coating-fluid solvent or a coating fluid produced by dissolving the charge transport substance is thereafter apt to suffer a deterioration, e.g., precipitation. In addition, there are cases where after application for photosensitive-layer formation, crystals separate out in the coating film and where the poor solubility results in a decrease in the efficiency of production of the coating fluid and production of photoreceptors.
In general, compounds in which the π-electron system has been expanded within the molecule tend to have higher interaction between molecules and lower solubility as the molecular size increases. Tetraphenylbenzidine frameworks have a large molecular size and tend to have low solubility, and when such tetraphenylbenzidine framework is substituted with a styryl group or the like to expand the π-electron system within the molecule, the molecule size is further increased, resulting in lower solubility in coating-fluid solvents. Because of this, contrivances for ensuring solubility, such as newly introducing a substituent and handling a substance as a mixture of geometrical isomers, are being made in the reports mentioned above.
However, those techniques, when used alone, have had a problem that the spread of π-electrons in the molecule of the charge transport substance is reduced due to the influence of the substituent introduced in order to improve solubility and a problem that since a desired structure has been intentionally isomerized to obtain a mixture of geometrical isomers in order to ensure solubility, the composition as a whole has an ionization potential which is higher than a desired value, resulting in an increase in residual potential after exposure.
Meanwhile, since electrophotographic photoreceptors are repeatedly used in an electrophotographic process, i.e., the cycle including charging, exposure, development, transfer, cleaning, and erase, the photoreceptors undergo various kinds of stress during the use and deteriorate. Examples of chemical deteriorations, among such deteriorations, include the damage caused to the photosensitive layer by the ozone, which has high oxidizing properties, and NOx which are generated, for example, from the corona discharge device that is commonly used as a charging device. There are cases where this damage results in a decrease in charging property. These troubles are largely attributable to the chemical deterioration of the charge transport substance contained in a large amount in the photosensitive layer.
As a result of increases in speed in electrophotographic processes, an increase in sensitivity and high-speed responsiveness have become essential. In addition to these requirements, a prolongation of the life of photoreceptors is desired from the standpoint of reducing the burden of maintenance of printers, copiers, and the like. Resistance to various kinds of stress in electrophotographic processes is also becoming highly important.